1. Field of the Invention
The present invention relates to a header transformation table used in transforming a header information of a cell in an ATM network, and a preliminary operation system including such a header transformation table which is to be provided between input transmission paths and an ATM switch in an ATM network, and which carries out a preliminary operation for appropriately transforming a cell format by using the header transformation table.
2. Description of the Background Art
As a conventional data transmission mode in a communication network such as a telephone network, a so called ATM (Asynchronous Transfer Mode) is known. In this ATM, the data are transmitted in a form of a short packet of a fixed length called a cell, and each terminal utilizes as much of the data transmission capacity of the network as necessary whenever the need for the transmission of cells arises. As a consequence, the ATM has an advantage of being capable of improving the transmission efficiency in the network.
In such an ATM network, it is necessary to provide a number of cell switches, each of which has a function of switching cells transmitted from a plurality of input transmission paths into a plurality of output transmission paths.
In addition, it is also necessary to provide a preliminary operation system between the input transmission paths and such a cell switch, which carries out a preliminary operation including the following operations with respect to each cell entering from the input transmission paths.
(1) A header transformation operation for transforming a header information of each cell by using a header transformation table.
(2) An operation for attaching a routing tag information indicating an output transmission path through which each cell is to be outputted.
(3) An operation for inserting and deleting idle cells.
(4) An operation for alleviating the timing requirements by using the internal buffers as an elastic store, that is an operation for elastic function.
However, a conventional preliminary operation system in a conventional ATM network has been associated with the following problems.
First, there are cases in which the header transformation table needs to be rewritten, so that when the header transformation operation is carried out by making an access to the header transformation table which happens to be in a process of being rewritten at a time of this access, the header transformation operation may not be carried out correctly and consistently and the erroneous cell transmissions may be caused in a conventional preliminary operation system.
Secondly, in order to recognize the idle cell, it has been necessary to look up the header information of each cell and compare that looked up header information with a prescribed idle cell header information pattern in a conventional preliminary operation system, such that additional hardware components are required for a comparator and a register to memorize the prescribed idle cell header pattern.
Similarly, the additional hardware components are necessary in a conventional preliminary operation system in order to carry out other cell type recognition operations similar to that of the idle cells described above, including a recognition of cells to be processed differently such as branching cells.